Powers of sqrt(18) rounded down.

Terms

a(0) =1a(1) =4a(2) =18a(3) =76a(4) =324a(5) =1374a(6) =5832a(7) =24743a(8) =104976a(9) =445375a(10) =1889568a(11) =8016758a(12) =34012224a(13) =144301645a(14) =612220032a(15) =2597429617a(16) =11019960576a(17) =46753733110a(18) =198359290368a(19) =841567195983